Camshaft phasers for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of the engine's camshaft. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type camshaft phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type camshaft phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, as required to meet current or anticipated engine operating conditions.
While vane-type camshaft phasers are effective and relatively inexpensive, they do suffer from drawbacks. First, at low engine speeds, oil pressure tends to be low, and sometimes unacceptable. Therefore, the response of a vane-type camshaft phaser may be slow at low engine speeds. Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump, therefore making it more difficult to quickly supply engine oil to the vane-type camshaft phaser. Third, using engine oil to drive the vane-type camshaft phaser is parasitic on the engine oil system and can lead to requirement of a larger oil pump. Fourth, for fast actuation, a larger engine oil pump may be necessary, resulting in additional fuel consumption by the engine. Lastly, the total amount of phase authority provided by vane-type camshaft phasers is limited by the amount of space between adjacent vanes and lobes. A greater amount of phase authority may be desired than is capable of being provided between adjacent vanes and lobes. For at least these reasons, the automotive industry is developing electrically driven camshaft phasers.
Harmonic gear drive units for angular positioning are also well known. Referring to FIGS. 1-2, a typical flat plate harmonic gear drive unit is shown as harmonic gear drive unit 10. Harmonic gear drive unit 10 comprises outer first spline 12 which may be either a circular spline or a dynamic spline (hereinafter referred to as circular spline 12) as described below; outer second spline 14 (hereinafter referred to as dynamic spline 14) which is the opposite (dynamic or circular) of circular spline 12 and is coaxially positioned adjacent circular spline 12; flexspline 16 disposed radially inwards of both circular spline 12 and dynamic spline 14 and having outwardly-extending flexspline teeth 18 for engaging inwardly-extending circular spline teeth 20 of circular spline 12 and also for engaging inwardly-extending dynamic spline teeth 22 of dynamic spline 14; and wave generator assembly 24 disposed radially inwards of and engaging flexspline 16.
Wave generator assembly 24 includes an elliptical shaped wave generator 26 with wave generator bearing 28 fitted tightly over wave generator 26 to conform to the elliptically shaped circumference of wave generator 26. Wave generator bearing 28 includes inner race 30, outer race 32, a plurality of balls 34, and ball cage 36. Inner race 30 is disposed immediately radially outward of wave generator 26. Outer race 32 is disposed radially outward of inner race 30 with the plurality of balls 34 disposed radially between inner race 30 and outer race 32. Balls 34 ride within inner race groove 38 formed in the outer circumference of inner race 30. Balls 34 also ride within outer race groove 40 formed in the inner circumference of outer race 32. Ball cage 36 includes annular section 42 from which a plurality of ball separators 44 extend axially such that adjacent balls 34 are separated by one of the plurality of ball separators 44. Ball separators 44 include arcuate surfaces 46 which are concave and face toward an adjacent ball separator 44 to follow the contour of balls 34. Arcuate surfaces 46 are concave such that the radius of each arcuate surface 46 is in a plane substantially parallel to annular section 42.
Flexspline 16 is a non-rigid ring which is fitted tightly over and elastically deflected by wave generator assembly 24. Flexspline teeth 18 have a slightly smaller pitch diameter than circular spline teeth 20.
Circular spline 12 is a rigid ring with circular spline teeth 20 engaging flexspline teeth 18. Circular spline 12 has slightly fewer circular spline teeth 20 than flex spline 16 has flexspline teeth 18. Circular spline 12 serves as the input member.
Dynamic spline 14 is a rigid ring with dynamic spline teeth 22 engaging flexspline teeth 18. Dynamic spline 14 has the same number of dynamic spline teeth 22 as flexspline 16 has flexspline teeth 18. Dynamic spline 14 rotates together with flexspline 16 in a one-to-one relationship and serves as the output member. Of course, the circular spline may have more teeth than the dynamic spline, in which case the rotational relationship is then reversed.
Dynamic spline 14 may be distinguished from circular spline 12 by chamfered corners 48 at its outside diameter. Chamfered corners 48 provide for visual distinction of circular spline 12 and dynamic spline 14 which may otherwise differ visually only by the number of circular spline teeth 20 and dynamic spline teeth 22 which may be difficult to visually discern. Alternatively, but not shown, dynamic spline 14 may not include chamfered corners 48, but rather circular spline 12 may included chamfered corners at its outside diameter.
Now referring to FIGS. 1, 2, 3, 3A, and 4, the elliptical shape of wave generator assembly 24 causes flexspline teeth 18 to engage both circular spline teeth 20 and dynamic spline teeth 22 along and near major elliptical axis Amajor in two mesh zones 180° apart and symmetrical to major elliptical axis Amajor as shown in FIGS. 3 and 3A. When wave generator 26 is rotated relative to circular spline 12 and dynamic spline 14, a rotation wave is generated in flexspline 16 (actually two waves 180° apart, corresponding to opposite ends of the major ellipse axis Amajor). Since dynamic spline 14 has the same number of dynamic spline teeth 22 as flexspline 16 has flexspline teeth 18, there is no relative rotation of flexspline 16 with dynamic spline 14. However, since circular spline 12 has slightly fewer circular spline teeth 20 than flexspline 16 has flexspline teeth 18, flexspline 16 rotates relative to circular spline 12. As a result, circular spline 12 rotates past the dynamic spline 14 during rotation of wave generator 26 relative to dynamic spline 14, defining a gear ratio therebetween (for example, a gear ratio of 50:1 would mean that 1 rotation of circular spline 12 past dynamic spline 14 corresponds to 50 rotations of wave generator 26 relative to dynamic spline 14). Harmonic gear drive unit 10 is thus a high-ratio gear transmission; that is, the angular phase relationship between circular spline 12 and dynamic spline 14 changes by 2% for every revolution of wave generator 26.
One type of electrically driven camshaft phaser being developed is shown in U.S. patent application Ser. Nos. 12/536,575; 12/844,918; 12/825,806; 12/848,599;12/965,057; 13/102138;13/112,199; 13/155,685; and U.S. patent application Ser. No. 13/184,975 which are commonly owned by Applicant and incorporated herein by reference in their entirety. This electrically driven camshaft phaser utilizes a harmonic gear drive unit to change the phase relationship between the crankshaft and a camshaft of the internal combustion engine. This is accomplished by connecting the crankshaft to the circular spline of the harmonic gear drive unit in a fixed ratio of rotation and by connecting the camshaft to the dynamic spline of the harmonic gear drive unit in a fixed ratio or rotation. In this way, when the wave generator of the harmonic gear drive unit of the wave generator is rotated relative to the circular spline and dynamic spline, relative rotation between the circular spline and dynamic spline is generated, thereby resulting in a change of phase relationship between the crankshaft and the camshaft.
Typical angular positioning applications of harmonic gear drive units are not constrained to limit the angular positioning range. However, when a harmonic gear drive unit is used in a camshaft phaser to change the phase relationship between the crankshaft and the camshaft, stop members may be employed to limit the phase authority in order to prevent over-advancing and over-retarding which may, for example, result in undesired engine operation and engine damage due to interference of the engine valves and pistons. An example of such stop members is shown in U.S. patent application Ser. No. 12/844,918. Testing has shown that if the stop members are engaged with a sufficient torque applied to the wave generator assembly, the splines of the harmonic gear drive unit may be damaged by wind-up of the flex spline.
Referring now to FIGS. 3, 3A, and 4 in which circular spline 12 is not visible and which represents a condition when the stop members (not shown) are engaged, circular spline 12 and the dynamic spline 14 of harmonic gear drive unit 10 are effectively locked relative to each other. Flexspline teeth 18, as always, maintain the two mesh zones with circular spline teeth 20 and dynamic spline teeth 22 symmetrical about major elliptical axis Amajor. As torque is continued to be applied to wave generator 26, flexspline 16 wants to continue driving circular spline 12 and the dynamic spline 14 in opposite directions, but cannot because circular spline 12 and the dynamic spline 14 are locked and therefore flexspline 16 is also rotationally locked. The torque on wave generator 26 is resolved into both a torque load through flexspline teeth 18, circular spline teeth 20, and dynamic spline teeth 22 in the two zones of mesh and also in a radial outward load where wave generator 26 is trying to deflect flexspline 16 radially outward where flexspline teeth 18 do not mesh with circular spline teeth 20 and the dynamic spline teeth 22. The torque load between flexspline teeth 18 engaged with circular spline teeth 20 and the dynamic spline teeth 22 resolves into a tangential force vector and a radial force vector, the radial force vector acting to push flexspline 16 away from circular spline teeth 20 and dynamic spline teeth 22 radially. This radial force tries to collapse flexspline 16 in the mesh zone. These two forces acting on flexspline 16 try to collapse flexspline 16 inward at the center of the mesh zone (at Amajor). Inward collapse of flexspline 16 is resisted by balls 34. If one of the balls 34 was perfectly in the center of the mesh zone, flexspline 16 would not be able to deflect. However, the natural orientation of balls 34 is to have one equal distance on each side of the center of the mesh zone. These forces are illustrated in FIGS. 3 and 3A. Under this condition, there are tangential force vectors which act to push each ball 34 adjacent each side of the center of the mesh zone apart as shown in FIG. 4 which shows annular section being deformed in an arched shape from its normally straight shape. Ball separators 44 prevent balls 34 from moving a significant amount, however, balls 34 apply a cantilever load on ball separators 44, causing ball cage 36 to move axially. Axial misalignment of balls 34 as a result of these conditions may increase the cantilever load on ball separators 44. As ball cage 36 moves axially, the cantilever load on ball separators 44 increases, causing annular section 42 to deform. It is this deformation of annular section 42 that allows wind-up of flexspline 16. This wind-up may lead to durability and operational issues of harmonic gear drive unit 10.
U.S. Pat. No. 3,285,099 shows an example of a wave generator bearing for a harmonic gear drive unit. In this example, a portion of the wave generator extends radially outward. When the inner race is placed on the wave generator, an inner race axial end of the inner race is positioned against the portion of the wave generator that extends radially outward. An annular section of a ball cage is placed proximal to and axially inward of the inner race axial end, defining an axial space between the annular section and the portion of the wage generator extending radially outward. Since the portion of the wave generator that extends radially outward is flush with the inner race axial end, it may not provide sufficient axial support to the ball cage.
What is needed is an eVCP which minimizes deformation of a ball cage of a harmonic gear drive unit and which minimizes wind-up of a flexspline of a harmonic gear drive unit.